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ES6 Reflect API Tutorial

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This post is a part 18 of ECMAScript 6 Complete Tutorial post series.

Object reflection is an language ability to able to inspect and manipulate object properties at runtime. JavaScript already had been supporting APIs for object reflection but these APIs were not organized under a namespace and also they threw exception when they fail to complete an operation. This made it difficult to write scripts involving object reflection.

ES2015 introduces a new object referred as Reflect which exposes methods for object reflection. These new methods don’t throw exception on failure rather they return error, which makes it easy to write code involving object reflection.

The Reflect Object

The Reflect object is not a constructor that is it we cannot use it with new operator. It’s a plain object that exposes the methods related to object reflection as its own properties. Reflect is the only object introduced by ES2015 Reflect API.

Only couple of major browsers support the Reflect object at the time of writing this article. Check out the compatibility here. We can use harmony-reflect polyfill created by Tom Van Cutsem to use Reflect in non-compatible browsers. If we are using a transpiler to convert ES2015 to ES5 then we don’t need the polyfill.

Reflect object exposes in total 14 methods for object reflection. Here is the list and explanation of each of the methods with examples:

Reflect.get(object, property[, this])

Reflect.get is used to retrieve value of an object property. The first parameter is the reference to the object and second parameter is the property name. If we are retrieving value of an accessor property then we can provide a third parameter which is the value of this keyword inside the get function.

Here is an example to demonstrate Reflect.get method:

var obj = {name: "narayan"};

var name = Reflect.get(obj, "name");

console.log(name); //Output "narayan"

The Reflect.get() method works the same way as the dot and brackets operators for retrieving value of an object property. The only reason why you would want to use Reflect.get() is when you want to retrieve value of an accessor property by providing a custom value for the this keyword inside the get function. manually.

Here is an example to demonstrate Reflect.set method:

var obj = {name: ""};

Reflect.set(obj, "name", "narayan");

console.log(obj.name); //Output "narayan"

The Reflect.set() method works the same way as the dot and brackets operators for setting value of an object property. The only reason why you would want to use Reflect.set() is when you want to set value of an accessor property by providing a custom value for the this keyword inside the set function. manually.

Reflect.defineProperty(object, property, descriptor)

Reflect.defineProperty is used to define an object property. Its same as the Object.defineProperty method. Reflect.defineProperty takes the same three arguments as the Object.defineProperty method.

Here is an example to demonstrate Reflect.defineProperty:

var obj = {};

Reflect.defineProperty(obj, "name", {value: "narayan"});

console.log(obj.name); //Output "narayan"

Reflect.deleteProperty(object, property)

Reflect.deleteProperty is used to delete an object property. It’s same as the delete operator. The first argument is the object reference and second argument is the property name.

Here is an example to demonstrate Reflect.deleteProperty:

var obj = {name: "narayan"};

Reflect.deleteProperty(obj, "name");

console.log(obj.name); //Output "undefined"

Reflect.has(object, property)

Reflect.has is used to find if an object property exists or not. It’s same as the in operator. The first argument is the object reference and second argument is the property name.

Here is an example to demonstrate the Reflect.has method:

var obj = {name: "narayan"};

console.log(Reflect.has(obj, "name")); //Output "true"

Reflect.has() is simply an alternate to in operator and they both can be used alternatively.

Reflect.getOwnPropertyDescriptor(object, property)

Reflect.getOwnPropertyDescriptor is used to retrieve descriptor of an object property. It’s same as the Object.getOwnPropertyDescriptor method. The first parameter is the object reference and the second parameter is the property name.

Here is an example to demonstrate the Object.getOwnPropertyDescriptor method:

var obj = {name: "narayan"};

var descriptor = Reflect.getOwnPropertyDescriptor(obj, "name");

console.log(descriptor);

Output is:

Object {value: "narayan", writable: true, enumerable: true, configurable: true}

Reflect.ownKeys(object)

The

Reflect.ownKeys

method returns an array whose values represent the keys of the properties of an provided object. It ignores the inherited properties. The only argument we need to pass is the object reference.

Here is an example to demonstrate Reflect.ownKeys method:

var obj = {name: "narayan", profession: "developer", __proto__: {author: "SitePoint"}};

var keys = Reflect.ownKeys(obj);

console.log(keys);

Output is:

["name", "profession"]

Reflect.preventExtensions(object)

Reflect.preventExtensions method is used to mark an object as non-extensible i.e., we cannot add new properties to it. It’s same as the Object.preventExtensions method.

Here is an example to demonstrate Reflect.preventExtensions method:

var obj = {};

Reflect.preventExtensions(obj);

obj.name = "narayan";

console.log(obj.name); //Output "undefined"

Reflect.isExtensible(object)

Reflect.isExtensible is used to check if an object is extensible or not. It same as the Object.isExtensible method. It takes only one argument that is the object reference.

Here is an example to demonstrate the Reflect.isExtensible method:

var obj = {};

Reflect.preventExtensions(obj);

console.log(Reflect.isExtensible(obj)); //Output "false"

Reflect.setPrototypeOf(object, prototype)

Reflect.setPrototypeOf is used to set [[prototype]] of an object. The first argument is the object reference and second argument can be either null or an object.

Here is an example to demonstrate the Reflect.setPrototypeOf method:

var obj = {};

Reflect.setPrototypeOf(obj, {name: "narayan"});

console.log(obj.name); //Output "narayan"

Reflect.getPrototypeOf(object)

Reflect.getPrototypeOf is used to retrieve the [[prototype]] of an object. The only argument it takes is the object reference.

Here is an example to demonstrate the Reflect.getPrototypeOf method:

var obj = {};

Reflect.setPrototypeOf(obj, {name: "narayan"});

console.log(Reflect.getPrototypeOf(obj));

Output is:

Object {name: "narayan"}

Reflect.enumerate(object)

Reflect.enumerate method takes an object as argument and returns an iterator object that contains the enumerable properties of the object. It also returns the enumerable inherited properties of an object.

Here is an example to demonstrate the Reflect.enumerate method:

var obj = {name: "narayan", profession: "developer"};

var iterator = Reflect.enumerate(obj);

console.log(iterator.next().value);
console.log(iterator.next().value);
console.log(iterator.next().done);

Output is:

name
profession
true

Reflect.apply(function[, this, args])

The

Reflect.apply

method is used to invoke a function with a given this value. The first argument is the reference to the function object, second argument is the value of this keyword and finally the third argument is an array whose values will be passed as arguments while invoking the function.

Here is an example to demonstrate the Reflect.apply method:

function function_name(x, y)
{
    return this + x + y;
}

var value = Reflect.apply(function_name, 30, [10, 20]);

console.log(value); //Output "60"

Reflect.construct(constructor[, args, prototype])

The Reflect.construct method is used to invoke a function as a constructor. The first argument is the function reference, second argument is an array whose values will be passed as the argument to the constructor and finally the third argument is another constructor whose prototype will be used as the prototype of the currently invoked constructor.

Here is an example to demonstrate the Reflect.construct method:

function constructor1(a, b)
{
    this.a = a;
    this.b = b;
    this.f = function(){
        return this.a + this.b + this.c;
    }
}

function constructor2(){}
constructor2.prototype.c = 30;

var obj = Reflect.construct(constructor1, [10, 20], constructor2);

console.log(obj.f()); //Output "60"

Why use the Reflect API?

Here are some of the reasons why we may want to use the Reflect API for object reflection:

In case we want to invoke a constructor with a prototype of another constructor then we would usually copy the original prototype property to a different variable and after invocation we would copy it back. But ES2015 makes it easy by providing the Reflect.constructor method which takes a prototype as its third argument. Here is an example to demonstrate this:

function constructor1(){}
constructor1.prototype.print = function(){
  console.log("Constructor 1");
}

function constructor2(){}
constructor2.prototype.print = function(){
  console.log("Constructor 2");
}

//In ES5

var tmpPrototype = constructor1.prototype;

constructor1.prototype = constructor2.prototype;

var obj1 = new constructor1();
obj1.print();

constructor1.prototype = tmpPrototype;

//In ES5
obj1 = Reflect.construct(constructor1, [], constructor2);
obj1.print();

Another case where we would find Reflect API useful is while using Object.defineProperty. It throws exception if it fails to do the operation but Reflect.defineProperty don’t throw exception instead it returns false which makes it easy to handle failure. Here is code example to demonstrate this:

//In ES5
var o = {};
Object.defineProperty(o, 'a', {
  get: function() { return 1; },
  configurable: false
});

try
{
    Object.defineProperty(o, 'a', { configurable: true });
}
catch(e)
{
    console.log("Exception");
}

//In ES2015
var o = {};
Reflect.defineProperty(o, 'a', {
  get: function() { return 1; },
  configurable: false
});

if(Reflect.defineProperty(o, 'a', { configurable: true }))
{

}
else
{
    console.log("Operation Failed");
}

Similarly none of the methods of the Reflect API throw exception.

Conclusion

In this tutorial, we learned what the object reflection is, and how to use the ES2015 Reflect API for the object reflection. We saw various methods of the Reflect object with examples. Overall, this article introduced the ES2015 Reflect API to inspect and manipulate the properties of objects.

Narayan Prusty
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    Narayan Prusty

    I am a full-stack web developer. I specialize in Blockchain and JavaScript. This is my personal blog where I write about programming and technologies that I learn and feel interesting to share.

    5 years ago 1 Comment Web Development
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